(1) Field of the Invention
The present invention relates to an arrangement for supplying communication lines with an operating current and more particularly to an arrangement to inductively couple this energy to the line.
(2) Description of the Prior Art
Traditionally, battery feed for a telephone subscriber's instrument has been provided via a relatively low resistance (about 400 ohms total) split winding transformer on the line side at the central office. This transformer feeds the loop current required by the telephone instrument from the "mid-point" of the split winding with a capacitor shunt for low a.c. impedance. The secondary of the transformer couples into the switching systems voice path. Because the transformer carries direct current, a physically large, expensive, and heavy device is required to maintain the high inductance necessary for a low insertion loss.
The telephone plant is frequently exposed to large longitudinal currents induced by commercial power lines. To prevent coupling of these currents into the metallic voice path, very good longitudinal balance is required. Achieving satisfactory balance and low susceptibility to 60 Hz induction is one of the more difficult tasks in line interface design. In the traditional method described, this requires careful transformer design with close matching of split winding characteristics. Separate means must also be provided for ON/OFF hookswitch sensing (loop supervision) apart from the battery feed scheme.
Recently, floating battery feed schemes have been proposed. An example of such a scheme is disclosed in the patent by L. Freimanis, et al., U.S. Pat. No. 4,056,691. In this approach, the battery feed current is coupled to the telephone instrument from a source of positive and negative potential that "floats," has very high impedance relative to earth ground. The intrinsic advantage of this method is that very good longitudinal balance, immunity to a.c. induction, is easily obtained. A measure of lightning transient protection comes naturally as a secondary benefit. In this method, the voice path is a.c. coupled to the "floating" line circuit which allows design of a much smaller, lighter coupling transformer. Also, loop detection of the ON/OFF state of the hook and dial pulses is possible via the same structure used to supply the floating d.c. potential.